When packing containers of a non-returnable type are manufactured through folding and heat-sealing of flexible packing laminate which comprises a carrier layer of paper, absorption of the contents at the edges is prevented by sealing off the cut edges which will be located inside the packing containers with the help of material strips of thermoplastic material which are heat-sealed to the inside of the packing material. In packing containers of the aseptic type or when for other reasons imperviousness of the container to gas and light is necessary, the strip cannot simply be manufactured from thermoplastic material but should also include a suitable barrier layer, e.g., an aluminum foil layer. Since contact with certain acid contents, e.g., fruit juices or wine, may involve precipitation of aluminum into the contents, the foil has to be wholly covered by layers of a thermoplastic material strip. Strips of this kind are know and comprise a relatively narrow layer of aluminum foil which on both sides is surrounded by layers of thermoplastic material whose edges extend beyond the edges of the aluminum foil and which are sealed in a liquid-tight manner to one another. Upon cutting of a material strip of this type, however, the aluminum foil is exposed once again o the cut surfaces and it has not been possible up to now to manufacture in a rational manner a strip of the required length in which an internal layer of barrier material, e.g., aluminum foil, is wholly enclosed by surrounding layers on all four edges of the strip.